Electronic organs, synthesizers and other electronic musical instruments employ piano-style keyboards wherein a pair of normally open electrical switch contacts is closed whenever a key is actuated. In this environment it is preferable that the switch close before the key completes its actuating stroke. As the key travels past the switch closure point, however, certain problems can arise.
In prior art key switch designs the key remains coupled to the moveable switch contact after switch closure occurs. As a result, travel of the key past the closure point (over-travel) presses the moveable contact against the fixed contact with a force which continues to increase until the key finally completes its excursion. For several reasons, this is not desirable.
While a certain level of contact pressure is of course essential to assure completion of the electrical circuit, it is not desirable for the contact pressure to increase greatly beyond that level. Additional contact pressure causes an excessive amount of contact wiping. Some wiping action is desired in an electrical switch mechanism, because it tends to scrub off surface impurities and thus promote bare metal contact. But if the wiping action occurs under excessive pressure, it results in rapid wear at the contact point. In the electronic musical instrument field, switch contacts are commonly clad or plated with precious metals to prevent corrosion and thus enhance circuit reliability. Wear eventually removes the precious metal layer from the surface of the contacts, and leaves them vulnerable once again to corrosion. If this wearing process is too rapid, it unduly shortens the useful life of the switch mechanism.
Then, too, excessive wiping action can cause problems in relation to the operation of the electronic circuitry of the instrument. Some of this circuitry is designed to respond to small input voltage levels received from the keyboard switches. Excessive wiping action, in conjunction with small surface asperities on the switch contacts, can produce multiple switch closures for a single key actuation, and the musical instrument then sounds the note a plurality of times (double keying), instead of only once as the musician intended. This is particularly a problem with certain types of electronic organ voices, such as "solo percussion."
After the key stroke is completed, if the musician continues to hold the key down in order to play a sustained note, he may move the key slightly from side to side while holding it down. If the key remains coupled to the moveable contact during hold-down, this sideward movement of the key may be transmitted to the moveable contact, which will then move back and forth across the surface of the fixed contact. Movements such as these cause wear, and can also cause signal interruptions. Double keying again results, causing the instrument to repeat the note rapidly instead of playing it only once for a sustained interval.
One way of dealing with these problems is to use a low resilience moveable contact member, such as a relatively limp coil spring. Such springs are so compliant that they minimize the above-described problems which occur during over-travel and hold-down of the keys. But the winding of a coil spring requires a considerable length of precious-metal-covered wire; and so this approach is costly.